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Browsing by Author "Scott, Brian Lee"

Now showing 1 - 3 of 3
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    Cure Kinetics of Two Part Epoxy Resin and the Effect on Characterization of Thermal Barrier Coatings
    Chang, Sunny (Virginia Tech, 2015-05-28)
    The aerospace industry strives to develop new methods of refining gas turbine engines by increasing power and thermal efficiencies while simultaneously reducing cost. Turbine engines operate under high temperatures and therefore thermal barrier coatings (TBCs) composed of yttria-stabilized zirconia (YSZ) play an important role in improving the performance of the components that make up the engine. Failure of the TBC could lead to catastrophic events, thus requiring consistent and accurate characterization for supplier qualification and production quality assurance. However, due to porosity and the anisotropic behavior of the coating and variability in processing of TBCs, consistent characterization has proven to be extremely challenging. One of the reoccurring issues is the inconsistency in measuring percent porosity, which stems from the difficulty in distinguishing filled pores from damaged, unfilled voids. Sample preparation of TBCs involves sectioning, mounting, grinding, polishing, and characterization. Eliminating variability in characterization begins with mounting which is a critical step to protect the surface integrity and edge retention of the coating during grinding and polishing. The curing kinetics of a slow cure two part epoxy was investigated and the TBC samples were mounted and cured at heating rates of 2, 5, and 10°C/min to 55°C and 70°C. Grinding and polishing procedures simulated industry practices followed by characterization with optical microscopy. Results showed that heating rates of 2°C/min to 55°C and 70°C have the best impregnation properties while uncontrolled or high heating rates of 10°C/min had an increase in the amount of pullouts and lack of infiltration from the epoxy. The curing kinetics of the epoxy needs to be controlled to eliminate the ambiguity of filled and unfilled pores.
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    Fabrication and Characterization of a Porous Clad Optical Fiber Gas Sensor
    Scott, Brian Lee (Virginia Tech, 2008-12-17)
    An optical fiber has been developed that can be used as a chemical gas sensor. Fabrication of the optical fiber produces a fiber that has a solid core with a porous cladding. The porous cladding region is made from a spinodally phase separable glass where the secondary phase is removed through dilute acid leaching. A non-phase separable glass composition is used for the core region. The properties of the phase separable glass are dependent on the processing conditions and the thermal history of the glass after the porosity has been achieved. Investigation of how processing conditions affected the pore structure was conducted to determine what pore characteristics are achievable for the glass composition used. Phase separation temperature, removal of silica gel deposited in the pores, and the post fabrication heat treating were used as experimental processing conditions. A maximum useable average pore size of approximately 29 nm was achieved. Maximum pore volume in the experimental groups was 0.4399 cc/g. Most heat treatments of the porous glass caused consolidation of the pore structure, with some conditions producing pore coarsening.
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    Semi-conductor Core Optical Fibers and Fabrication Dependence of the Grain Structure
    Scott, Brian Lee (Virginia Tech, 2011-09-09)
    The production and fabrication of semi-conductor core optical fibers was shown to be feasible and controllable. This was accomplished through the step sequence of fabrication and characterization of 4 fiber types, an experiment on controlling the grain length in the core and a simple model of the heat transfer during fabrication. Fibers were first made with a silicon core, followed by a phosphorous doped n-type silicon core, then a boron doped p-type silicon core, and a tellurium doped n-type gallium antimonide core. Characterization of the fibers was accomplished with energy dispersive spectroscopy (EDS) for compositional analysis, electron backscatter diffraction (EBSD) for crystal orientation and grain size, optical and electron microscopy for physical fiber quality and optical transmission for core optical quality. A model was developed to relate the heat transfer with the grain structure of the fiber core. All of the fibers fabricated had a polycrystalline core with either no detectable oxygen in the case of the silicon fibers or low amounts of oxygen diffusion into the core as in the case of the GaSb fibers. Fiber lengths ranged from 7 cm for the initial silicon fibers to 60 cm and outside diameters down to 100 µm for n and p type silicon fibers. Core diameters for all fiber types ranged from 10 – 200 µm depending on the fabrication parameters. Lengths of major grains in the core are dependent on the core diameter and the pulling speed. The grain lengths of the major grains in the core generally increase in length with an increase in core diameter. Grain lengths in all fibers are thought to be suitable for use in fabrication of electronic structures in the core region with even the smallest average grain length of around 300 µm. This grain structure satisfies the grain boundary requirements for fabrication of boundary free p-n junctions and other more complicated electronic structures. Small core diameter fibers had better physical quality with fewer cracks and longer continuous length than the larger core fibers.